Bifenthrin Susceptibility Among Western Corn Rootworm (Diabrotica virgifera virgifera LeConte) Field Populations

Because Western Corn Rootworm (Coleoptera: Chrysomelidae) has evolved resistance to many insecticides, this research was aimed at determining the baseline susceptibility for the pyrethroid insecticide, bifenthrin, from unselected laboratory populations and to compare the baseline with field populations. Nine unselected laboratory colonies from USDA-Brookings, SD, and one non-diapause colony from commercial vendor were received. Adults from each colony were exposed to six bifenthrin concentrations plus control to estimate the lethal concentration that killed 50% (LC₅₀) of the sample tested using glass vial exposures. Ten unsexed WCR adults were added to each vial and each concentration was conducted in triplicate. The combination of the data from all ten colonies was used to calculate an overall LC₉₉ (0.8 µg/vial) that was used as a diagnostic concentration to examine susceptibility of field populations. In 2013, 29 field populations from nine different US states were collected and tested at the diagnostic concentration (10 beetles per vial with 10 replications per collection site). Reduced susceptibility was observed in more western populations especially in Perkins Co, Nebraska, and Deerfield Co, Kansas with only 20% and 22% adult mortality, respectively. For the other populations tested, mortality of WCR adults ranged between 40% and 100%. The areas where reduced susceptibility was observed correspond to areas where bifenthrin is commonly used for adult control. Because these collections may have occurred after treatment, we will attempt to obtain future collections from these areas both before and after insecticide application.